Various instances of such systems are known, for example from Australian patent 658857 of Marsh and Lenarcic or Australian patent 664544 of Turner and Cole. In some variations of the system the interrogation medium may be other than electromagnetic, such as optic or acoustic.
The present invention is related to and may include developments disclosed in applicant's PCT patent applications AU92/00143, AU92/00477 and AU98/00017, the disclosures of which are incorporated herein by cross-reference.
Most of the features desirable in multiple read systems may be satisfied by allowing two-way communication between the interrogator and the labels. In contrast to simple schemes in which the interrogator merely provides an energising signal for the label, or a trigger signal telling the labels when to begin a sequence of replies, in this disclosure the interrogator, in addition to receiving and decoding replies from labels, can send to the labels a limited number of information bearing signals, and the labels can act upon those signals in simple but useful ways.
The are a number of factors which should be taken into account in the design of an RFID system, and in consequence in the design of a signalling system therefor. These are listed below.
Often labels may traverse rapidly through the field. This may create a need for fast data rates of communication from the label, and a need to ensure that the circuits used within the label are robust with respect to the variations in excitation levels, over the time available for its reading. There are consequences for the communication protocols which should be used in these situations. It is not guaranteed that all labels are within the field for roughly the same length of time.
Frequently, there is no guarantee that the labels are well coupled to the particular field orientation provided. In practical situations, there is a large range of field strengths to which labels are exposed. In addition, the small signal suppression effect must be taken into account in determining whether some label responses have been masked by other responses.
A wide variety of applications to which high frequency RFID labels are to be put have been identified in the market place.
Different applications place emphasis on different aspects of performance such as: reading range; reading speed; code capacity; manufacturing cost; time for which labels are in the interrogation field; throughput in terms of number of labels read per second; capacity to read against environmental noise; the need for extended label features such as combined theft detection and data recording capability; performance in a range of EMC environments; coping with a range of couplings between label and interrogator; and survival of the reading process during temporary field extinguishment or variation of field direction.
An interrogator—label signalling system must be designed with the flexibility that will cater, in an orderly way, for labels with the desirable range of functionality which meets these diverse operational demands.